It is well known that arterial blood pressure varies quasi-periodically between systolic (highest) and diastolic (lowest) pressure as a consequence of pumping action of the heart. Blood vessels are elastic ducts, made of viscoelastic materials, which expand and contract in unison with the arterial blood pressure. Arterial walls move radially and arterial lumen increases in volume as the heart pumps and decreases in volume between beats. The lumen of the arteries is filled with blood, so that the volume of blood within a section of the body (such as chest or a limb) also varies in unison with the arterial blood pressure.
The expansion and contraction of arterial walls may be detected in many ways: by ultrasound, by x-rays, by electromagnetic imaging, by mechanical devices, by impedance plethysmography and by colorimetry.
Physicians have long sought a safe, noninvasive method for continuous monitoring of arterial blood pressure (ABP) in critical care patients because ABP is the major vital sign indicator. They are currently doing this invasively by sticking probes (needles or catheters) into large arteries. The probes are connected to pressure transducers which transform the detected pressures into images of the waveform, or into a set of numbers indicating the blood pressure variation. The procedure is painful to the patient, cumbersome for medical personnel, and potentially unsafe. It may cause hematoma, damage to the artery, blood clotting and infection.
Any device, capable of providing the same useful service but noninvasively without puncturing, physically penetrating or otherwise harming the patient would be vastly preferable.
There is currently on the market a device called Finapress (manufactured by Ohmeda) which successfully accomplishes the task. A cuff is linked to a control system which contains two essential elements: a color detector and a fast responding pressure-regulating device. The operating principle is to balance the transmural (arterial blood) pressure at all times and restrict the movement of the arterial walls to a minimum. The control loop begins with the very sensitive color detector which detects minute changes of color of the tissues under the cuff, caused by the tendency of the blood to accumulate or decumulate during the blood pressure variation. The detected change provides a command signal to the pressure regulating device to pneumatically adjust the pressure in the cuff, to counteract the transmural arterial pressure variation. This counteracting pressure, which is very similar to the arterial pressure, is then displayed continuously, in real time, as the monitored arterial blood pressure waveform.
The device is complicated and has a very serious limitation: it can operate only on a fingertip, because its mechanics requires a small amount of transparent soft tissue backed by solid bone structure. Unfortunately, the blood flow through the tip of a finger is first to be shut off by the circulating system when a patient approaches shock conditions. Hence, Finapress becomes useless when needed the most. Therefore, a more suitable device, capable of working under all conditions, including shock conditions, would be a substantial improvement in accomplishing the task of reliably, noninvasively and continuously monitoring blood pressure.
U.S. Pat. No. 3,920,004 describes a noninvasive blood pressure sensor utilizing blood flow volume measurements. U.S. Pat. No. 3,996,924 measures venous patency of a human limb by measuring the venous outflow within a defined time interval after release of a forced blockage utilizing electrical impedance measurements. U.S. Pat. Nos. 3,996,925, 4,437,469 and 4,562,843 describe systems for determining characteristics of blood flow; however, there is no description of a device capable of noninvasive continuous blood pressure measurements.